INCREASED VERTEBRAL BODY TETHER TENSION RESULTS IN GREATER SPINAL GROWTH MODULATION

Increased vertebral body tether (VBT) tension has been shown to increase spinal growth modulation in an in-vivo porcine model. However, this relationship has not been studied in growing children. The aim of this study was to compare vertebral growth between the tethered and untethered side of individual vertebral bodies with varying amounts of tether tension. A multicenter pediatric registry was queried for patients with idiopathic scoliosis who were treated with right-sided VBT, had intraoperative tension values, and 3D reconstructions from biplanar radiographs at the first postoperative (FE) and 2yr time points. Tension was recorded as ordinal values and grouped as No Tension, Slight-Moderate, Maximal. Custom Matlab code was used to calculate vertebral height (mm) on the concave (untethered) and convex (tethered) sides of each vertebra from T5-T12 (most consistently instrumented in this cohort). Generalized linear mixed models (with subject as a random factor and one interaction term between intervertebral tension and the vertebral level above the tensioned segment) were used to analyze the effect of tension on vertebral body growth over the postop period. Fifty-two subjects with 330 instrumented vertebral bodies met inclusion. Mean age was 12.5±1yrs; 47F, 5M. The majority were skeletally immature (23 open TRC, 29 Closed; 38 Risser 0, 6 Risser 1, 8 Risser 2-4; Proximal Femoral Maturity Index (PFMI) 2: 9, 3: 21, 4: 20, 5: 2). The average height change from FE to two years was 1.6±1.9mm on the concave side, 1.2±1.8mm on the convex, and 0.3±2.1mm of differential change (concave-convex). No difference was found in concave height change based on tension (p=0.2). On the convexity, greater tension resulted in less height change (all groups differed; p≤0.04). Greater differential growth was observed with maximal tension than with no tension (p In this cohort of largely skeletally immature patients treated with VBT, the amount of intervertebral tension applied intra-operatively had a significant impact on vertebral body growth over two years, with greater tension resulting in less convex-sided vertebral growth and greater differential growth between the concave and convex sides of the vertebra.